The invention relates generally to methods and reactors for cracking hydrocarbon and methods for coating the reactors. More specifically, the invention relates to methods and reactors for cracking hydrocarbon, in which the build-up of coke deposits are undesirable.
In the petrochemical industry, hydrocarbons such as ethane, propane, butane, naphtha and gas oil are cracked in reactors, in the presence of from about 30 weight percentage (wt %) to about 70 wt % of steam, at temperature of from about 700° C. to 870° C. in order to produce light olefins such as ethylene, propylene and butene. Sometimes, hydrocarbons such as bottoms from atmospheric and vacuum distillation of crude oil are cracked in reactors at a temperature in a range from about 480° C. to about 600° C. in the presence of about 1 wt % to about 2 wt % steam to produce light hydrocarbon fractions and coke.
The reactor is usually a pyrolysis furnace comprising a firebox through which runs an array of tubing. The array of tubing and corresponding fittings may total several hundred meters in length. The array of tubing may comprise straight or serpentine tubes.
During hydrocarbon cracking processes, the build-up of carbonaceous deposits (i.e. coke deposits) usually happens on inner surfaces of reactor components, for instance, inner radiant tube surfaces of furnace equipment. The inner radiant tube surfaces become gradually coated with a layer of coke, which raises the radiant tube metal temperature (TMT) and increases the pressure drop through radiant coils. In addition, coke build-up adversely affects the physical characteristics of the reactor components, such as the radiant tubes, by deteriorating mechanical properties such as stress rupture, thermal fatigue, and ductility due to carburization.
In order to decoke reactor components, the hydrocarbon cracking must be periodically stopped. Typically, the decoking is carried out by combustion of the coke deposits with steam/air at temperatures of up to 1000° C. Such decoking operations are required approximately every 10 to 80 days, depending on the operation mode, types of hydrocarbons and hydrocarbons throughput, and result in production loss since hydrocarbons feeding must be stopped for such decoking operation.
A variety of methods have been considered in order to overcome the disadvantages of coke build-up on reactor components, such as furnace tube inner surfaces. These methods include: metallurgy upgrade to alloys with increased chromium content of the metal substrates used in the furnaces; adding additives such as sulfur, dimethyl sulfide (DMS), dimethyl disulfide (DMDS) or hydrogen sulfide to the feedstock; and increasing steam dilution of feedstock.
While some of the aforementioned methods have general use in the petrochemical industry, it is desirable to provide a new method and reactor that obviates and mitigates the shortcomings of the prior art and successfully reduces or eliminates the build-up of coke deposits.